混叠源表

Question

是否有方法在单个作用域的上下文中对源表进行别名？

我试过这个：

scope = User.all
scope.arel.source.left.table_alias = "toto"
scope.where(firstname: nil) => "SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`firstname` IS NULL"

问题是，模型类为所有后续查询保留别名：

User.all => "SELECT `toto`.* FROM `users` `toto`"

编辑

我将此方法添加到ApplicationRecord中

def self.alias_source(table_alias)
  klass = Class.new(self)
  klass.all.source.left.table_alias = table_alias
  klass
end

现在，我可以：

User.alias_source(:toto).where(firstname: nil) => "SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`firstname` IS NULL"

Answer 1

因为创建继承自ActiveRecord::Base的匿名类会导致内存膨胀，所以我不确定我会推荐它(参见此处：https://github.com/rails/rails/issues/31395)。

也许更好的实现是以块形式执行，例如将别名表交给块，然后将其设置回块。

例如：

def self.with_table_alias(table_alias, &block)
  begin 
    self.all.source.left.table_alias = table_alias
    block.call(self)
  ensure 
    self.all.source.left.table_alias = nil 
  end
end

用法

User.with_table_alias(:toto) do |scope| 
  puts scope.joins(:posts).where(firstname: "Ruur").to_sql
end 
# "SELECT `toto`.* 
#  FROM 
#    `users` `toto` 
#    INNER JOIN `posts` ON `posts`.`user_id` = `toto`.`id` 
#  WHERE 
#    `toto`.`firstname` = 'Ruur'" 

puts User.all.to_sql
# "SELECT `users`.* FROM `users`

警告:如果没有对边缘案例和其他进行广泛的测试，就不会在生产环境中对此进行测试。

更新以解决所需的实现

module ARTableAlias

  def alias_table_name=(val) 
    @alias_table_name = val 
  end 

  def alias_table_name 
    @alias_table_name ||= "#{self.table_name}_alias" 
  end   

  def alias_source
    @alias_klass ||= Class.new(self).tap do |k|
      k.all.source.left.table_alias = alias_table_name
    end  
  end 
end 

然后以下列方式使用：

class User < ApplicationRecord 
  extend ARTableAlias 

  alias_table_name = :toto
end 

User.alias_source.where(first_name = 'Ruur') 
#=> SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`first_name` = 'Ruur'
User.where(first_name = 'Ruur') 
#=> SELECT `users`.* FROM `users` WHERE `users`.`first_name` = 'Ruur'
User.alias_source === User.alias_source
#=> true

Answer 2

我将此方法添加到ApplicationRecord中

def self.alias_source(table_alias)
  klass = Class.new(self)
  klass.all.source.left.table_alias = table_alias
  klass
end

现在，我可以：

User.alias_source(:toto).where(firstname: nil) => "SELECT `toto`.* FROM `users` `toto` WHERE `toto`.`firstname` IS NULL"